Inorganic Chemistry: Qualitative Analysis

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Group II A and Group II B cations

Group II A - Hg²⁺, Cu²⁺, Bi³⁺, Cd²⁺, Pb²⁺
Group II B - As³⁺, As⁵⁺, Sb³⁺, Sb⁵⁺, Sn²⁺, Sn⁴⁺

Hg₂Cl₂ with NH₃

Hg↓ + HgNH₂Cl↓(black)

Hg₂²⁺ reaction with KI

Forms green Hg₂I₂↓ which in excess disproportionates to Hg + [HgI₄]²⁻

Gas released when Pb reacts with moderately concentrated HNO₃

NO is released

Pb²⁺ reaction with NaOH

Pb(OH)₂↓ which dissolves in excess NaOH

Pb²⁺ with Na₂SO₃

PbSO₃↓ which is less soluble than PbSO₄ but dissolves in acidic or basic medium

Hg₂²⁺ reaction with sulphide ion

Hg↓ + HgS↓ , where HgS dissolves in excess Na₂S

Hg₂(NO₃)₂ with NH₃

Hg↓ + HgO•Hg(NH₂)(NO₃)↓ (black)

Hg₂²⁺ with SnCl₂

Hg + Sn⁴⁺

AgCl readily dissolves in

NH₃, CN⁻ or S₂O₃²⁻ solutions

Colours of sulphides of group IIA cations

PbS, CuS - Black
CdS - yellow
Bi₂S₃ - Brownish-black

Colours of Group IIB sulphides

As₂S₃, As₂S₅ - yellow
Sb₂S₃, Sb₂S₅ - orange
SnS - brown
SnS₂ - yellow

Hg²⁺ with KI

It forms red ppt of HgI₂ which dissolves in excess KI to form Nessler's reagent K₂[HgI₄]

Hg²⁺ with SnCl₂

It gets reduced to Hg₂Cl₂.
If excess is added, it is further reduced to Hg

Reaction of Bi³⁺ with NH₃

Bi(OH)₂NO₃↓ (white ppt)

NH₃ is added to Cu²⁺

Cu(OH)₂•CuSO₄↓(light blue ppt) which dissolves in excess NH₃ to form Cu[(NH₃)₄]²⁺ (Schweizer's reagent)

Cu²⁺ with KI

Cu₂I₂↓ and intense brown I₃⁻ ions

Cu²⁺ with KCN or KSCN

SCN⁻ shows similar reaction except Cu(SCN)₂ is black in colour

Cd²⁺ with KI

No precipitate

Cd(OH)₂ and Cd(CN)₂ color

white ppt

colours of FeS, Al(OH)₃, Cr(OH)₃, NiS, CoS, MnS, ZnS

iron(II) sulphide (black),
aluminium hydroxide (white),
chromium(III) hydroxide (green),
nickel sulphide (black),
cobalt sulphide (black),
manganese(II) sulphide (pink),
zinc sulphide (white)

Decomposition of K₄[Fe(CN)₆] in hot conc H₂SO₄

K₄[Fe(CN)₆] + 6H₂SO₄ + 6H₂O → FeSO₄ + 2K₂SO₄ + 3(NH₄)₂SO₄ + 6CO↑

Fe²⁺ with K₄[Fe(CN)₆]

K₂Fe[Fe(CN)₆]↓ (pale-blue ppt)

Cu²⁺ with K₄[Fe(CN)₆] + CH₃COOH

Cu₂[Fe(CN)₆]↓ (chocolate-brown ppt)

As³⁺ with ammonium molybdate

Canary yellow ppt

Fe³⁺ with H₂S in acidic medium

2Fe³⁺ + H₂S → 2Fe²⁺ + S↓

Fe³⁺ with K₃[Fe(CN)₆]

Fe[Fe(CN)₆] (brown colouration)

Fe³⁺ with CH₃COONa

Note : FeCl₃ is used in detection of CH₃COOH ion and similar reaction occurs

Fe³⁺ with KSCN

Fe(SCN)₃ (blood red colour)

Lake test for aluminium

When blue litmus is added to the solution, a red
colouration is obtained due to the acidic nature of the solution. On addition
of NH4OH solution drop by drop the solution becomes alkaline and
aluminium hydroxide is precipitated. Aluminium hydroxide adsorbs blue
colour from the solution and forms insoluble adsorption complex named
‘lake’. Thus a blue mass floating in the colourless solution is obtained. The
test is therefore called lake test.

Cr³⁺ in NaOH

Cr(OH)₃↓ which dissolves to form [Cr(OH)₄]⁻ (green solution)

Which cations give precipitate with NaOH but are soluble in excess NaOH?

Pb²⁺ - [Pb(OH)₄]²⁻
Zn²⁺ - [Zn(OH)₄]²⁻
Al³⁺ - [Al(OH)₄]⁻
Sb³⁺ - SbO₂⁻
Sn²⁺ - [Sn(OH)₄]²⁻
Sn⁴⁺ - [Sn(OH)₆]²⁻
Cr³⁺ - [Cr(OH)₄]⁻
Ga³⁺ - [Ga(OH)₄]⁻

Colour of Co²⁺

In the aqueous solutions of cobalt(II) compounds the red Co²⁺ ions are
present. Anhydrous or undissociated cobalt(II) compounds are blue.

Solubility of CuS in concentrated H₂SO₄ and concentrated HNO₃ separately

Insoluble in H₂SO₄.
Leaves white ppt of S but dissolves in conc. HNO₃

reaction of Ni²⁺ with NH₃

Ni(OH)₂↓(Green) which in excess NH₃ forms [Ni(NH₃)₆]²⁺ (deep blue complex)

Ni²⁺ with DMG

Specific test for Ni²⁺
Rosy red ppt

Mn²⁺ with PbO₂ or S₂O₈²⁻

Pink colour is obtained due to formation of MnO₄⁻

Zn²⁺ with K₄[Fe(CN)₆]

K₂Zn₃[Fe(CN)₆]₂↓ (white ppt)
soluble in alkali but insoluble in dilute acids

Rinmann's green test, specific for zinc

When a salt of zinc is strongly heated with CoNO₃ , a green colored bead is formed.
It is called Rinmann's green with formula majorly ZnO•CoO

Ba²⁺ with C₂O₄²⁻

BaC₂O₄↓ which dissolves in hot dil. acetic acid(distinction from calcium)

Ba²⁺ in K₂CrO₄

BaCrO₄↓ (yellow ppt)
this reaction is performed in acetic acid to prevent the precipitation of SrCrO₄ and CaCrO₄

Sr²⁺ with H₂SO₄ and how to distinguish from Ca²⁺

SrSO₄↓ which is insoluble in (NH₄)₂SO₄(distinction from calcium)

Ca²⁺ with C₂O₄²⁻

CaC₂O₄↓(insoluble in acetic acid)
but readily soluble in mineral acids

Mg²⁺ with Na₂HPO₄, NH₄Cl and other ammonia solutions

Mg(NH₄)PO₄•6H₂O↓(white ppt)

Heat AgNO₃

450°C → AgNO₂ + 1/2O₂ → Ag + NO₂ + 1/2O₂(strong heating)

Heat CuCl₂

Cu₂Cl₂ + 1/2Cl₂

Heat PbO₂

PbO + 1/2O₂

Heat Fe₂(SO₄)₃

Fe₂O₃ + SO₃

Heat FeSO₄

Fe₂O₃ + SO₂ + SO₃

Heat Na₂S₂O₃

Na₂SO₄ + Na₂S₅ + H₂O

Heat Hg(NO₃)₂

Hg + NO₂ + 1/2O₂

Heat Cu(NO₃)₂

CuO + NO₂ + 1/2O₂

Heat 2MCl₃•6H₂O where is Fe, Cr, Al

M₂O₃ + 6HCl + 6H₂O

Heat KMnO₄

K₂MnO₄ + MnO₂ + O₂

Heat Na₂SO₃

Na₂SO₄ + Na₂S

Complex formed in brown ring test

[Fe(H₂O)₅NO]SO₄

S₂O₃²⁻ with AgNO₃

Ag₂S₂O₃↓ which dissolves in excess [Ag(S₂O₃)₂]³⁻ complex

nitroprusside test of S

Na₂[Fe(CN)₅NO] + Na₂S → Na₄[Fe(CN)₅NOS]
violet colour is formed

NO₂⁻ with sulphamic acid

N₂ is released (sulphamic acid is HO•SO₂•NH₂)

Sulphides soluble in concentrated HCl

Sb₂S₃, SnS, SnS₂, CdS, Bi₂S₃, FeS, MnS, ZnS
Sb₂S₅ is soluble but leaves white ppt of S.
BTW, PbCl₂ is also soluble in conc HCl

Sulphides not soluble in concentrated HCl

As₂S₃, HgS, CoS

Sulphides soluble in Hot concentrated HNO₃

Ag₂S, PbS, CuS, CdS, Bi₂S₃, NiS, CoS, MnS, ZnS (they dissolve but leave white ppt of S which disappears onprolonged heating due to formation of SO₄²⁻)
As₂S₃ dissolves completely

Sulphides insoluble in concentrated HNO₃

HgS

Colour of Hg₂I₂ and Hg₂CrO₄

Green and red respectively

Ammonium molybdate

(NH₄)₂MoO₄

Ammonium phosphomolybdate

(NH₄)₃PO₄•12MoO₃

Ions which form complex with NH₃

Ni²⁺ , Cd²⁺ , Zn²⁺ , Ag⁺ , Cu²⁺